1. Field of the Invention
The present invention relates generally to an apparatus and a method for providing a uniform, consolidated, unidirectional, continuous, filament-reinforced polymeric material. In particular, it relates to an apparatus and a method using a pre-melting chamber, a stationary bar assembly, and a loaded, cooled nip-roller apparatus.
2. Description of the Related Art
Consolidation of voidy prepreg materials has been accomplished by pultrusion. Consolidation is defined as the elimination of voids during meltprocessing. This technique requires full ingestion of the unconsolidated prepreg within an enclosed die with an exit area less than the inlet area. Within the heated closed die, processing of the polymer forces the polymer melt to flow axially to the filament array, whereas flow transverse to the filament array is generally 1/10 to 1/100 of the axial flow. As a consequence of the geometry and boundary limits of the pultrusion die, voids must be expelled axially, against the flow of the prepreg via the entrance of the pultrusion die. This complex flow of voids is known to limit the rates at which pultrusion may proceed. With the desirable prepreg attribute of low void content, the pultrusion process is limited in the length of the die because the longer the die, the longer the voids must travel to be fully expelled. This contributes to a very slow production rate.
In U.S. Pat. No. 4,680,224 to O'Conner, a pultrusion process is disclosed wherein a poly(arylene sulfide) polymer matrix system is consolidated. O'Conner specifies pultruding powder-impregnated glass rovings and pultruding aqueous slurry impregnated fiber strands with a commercially available pultrusion apparatus where the production rates achieved 15 cm/minute. O'Conner states that a major operational problem of pultrusion is encountered at the die entrance, wherein fiber jamming was explained to cause catastrophic failure of the pultrusion process.
U.S. Pat. No. 2,702,408 to Hartland discloses a pultrusion apparatus with two separate dies, where one is heated and the other is cooled. U.S. Pat. No. 4,820,366 to Beever et al. discloses the further development of this concept to include a means of impregnation of dry filaments. However, this process is deficient because the process must be stopped occasionally to open the cooling die and massage the material and remove fiber balls.
O'Conner in U.S. Pat. No. 4,883,552 discloses a pultrusion process wherein 1/4 inch diameter carbon fiber rods made from a carbon filament array slurry powder impregnated with polyphenylene sulfide polymer is pultruded. Production rates were noted at 15 inches per minute which are considered to be very slow.
The design of the present invention does not exhibit sensitivity to fiber jamming because no rigid die entrance is involved. Anomalies such as fiber balls and dry filament areas on the unconsolidated prepreg are allowed to pass freely through the processing apparatus and do not cause catastrophic failure of the process. By the present invention, processing rates for slurry powder coated polyimide thermoplastics have been achieved as high as 50 feet/minute for sustained time intervals of 45 minutes. Although viscous flow is commonly known to be faster axially than transverse to a filament array (estimated by a bank of adjacent cylinders), void expulsion occurs transverse to the filaments in the present invention because the unconsolidated prepreg material is flattened out which provides a short transverse distance for the voids to translate. Pultrusion facilitates void expulsion in the axial direction only, it cannot facilitate void expulsion in the transverse direction because the mold is solid at the top, bottom and two sides. In terms of required void translation distances, for the present invention, an axial void translation distance of 0.01 inch and a transverse distance of 0.005 inch are required, whereas for pultrusion an axial void translation distance of 4 inches is common.